1. Field of the Invention
The present invention relates to high frequency DC-to-AC switchmode power converters and specifically to self-oscillating half-bridge inverters. More specifically, the present invention relates to a high frequency ballast for high intensity discharge lamps.
2. Prior Art
Self-oscillating DC-to-AC inverters have a significant position in the field of switchmode power converters, due to their simplicity and usefulness. Generally, DC-to-AC inverters are configured as push-pull, half-bridge or full-bridge. One of the simplest, and oldest, DC-to-AC self-oscillating push-pull inverters is the Royer circuit. Another circuit similar to the Royer circuit, which removes the switch drive function from the main power transformer, is a self-oscillating voltage or current driven Jensen circuit. An improved version of the Jensen circuit is described in U.S. Pat. No. 4,935,673 assigned to the assignee of the present invention, showing a current driver push-pull converter used with HPS lamps. A special half-bridge configuration which can be used with HID lamps is U.S. Pat. No. 5,097,183 also assigned to the assignee of the present invention, including two specially connected half-bridge inverters as a self-oscillating master and a controlled slave. The common disadvantage of the push-pull configurations is the imbalance problem of the push-pull transformer, especially when it is applied to asymmetrical loads.
An important application of the simple self-oscillating DC-to-AC switchmode power inverter is to provide DC-to-DC transformers which are widely used in the area of DC power supplies.
Another important application of the simple self-oscillating DC-to-AC switchmode power inverter, is supplying high intensity discharge (HID) lamps, especially high pressure sodium (HPS) lamps in the range of 35 to 400 watts. In this case, the load impedance of the DC-to-AC inverter is a HID lamp connected in series with an inductor. In the case of a high frequency powering of the HID lamps, the interaction between the high frequency ballast and the lamp is stronger then that of a conventional ballast (acoustic resonance). This high frequency ballast is significantly better then a conventional ballast due to its lessened weight and higher efficiency. Additionally, the high frequency ballast, utilized with an HPS lamp would have a longer life time, exhibit somewhat better light efficiency and display a better color temperature.
Therefore, the critical design targets for high frequency ballasts supplying HPS lamps would be the following:
(a) very high efficiency (energy saving);
(b) ensuring that the lamp power is maintained constant during the lifetime of the lamp at .+-.15% input voltage fluctuation;
(c) eliminating acoustic resonances by using frequency modulation;
(d) providing high voltage (.apprxeq.3000 V) ignition pulses;
(e) the relative simplicity of the ballast which would result in a lower cost; and
(f) reliability and longer lifetime.